Integrated circuits are very complex devices that include multiple layers. Each layer may include conductive material, isolating material and/or semi-conductive material. These various materials are arranged in patterns, typically in accordance with the expected functionality of the integrated circuit. The patterns also reflect the manufacturing process of the integrated circuits.
Conductive layers typically include conductors made of conductive materials, wherein the conductors are separated by isolating materials such as various oxides. The dielectric layers are located between the conductive layers in an interlaced manner. Conductors of distinct conductive layers may be connected to each other and/or to the substrate by conductive materials (termed interconnects or vias) located within the dielectric layers. The substrate may include semi-conducting materials and at least a portion of the substrate is connected to a virtual ground.
Various inspection and failure analysis techniques have evolved for inspecting integrated circuits both during the fabrication stages, between consecutive manufacturing stages, either in combination with the manufacturing process (also termed “in line” inspection techniques) or not (also termed “off line” inspection techniques). Various optical as well as charged particle beam inspection tools and review tools are known in the art, such as the Compluss™, SEMVision™ and Insite™ of Applied Materials Inc. of Santa Clara, Calif.
Manufacturing failures may affect the electrical characteristics of the integrated circuits. Some of these failures result from unwanted disconnections between various elements of the integrated circuits. An under-etched via or conductor can be floating instead of being connected to a conducting sub-surface structure.
Such a failure can be detected due to charging differences between the defective structure and non-defective structures. In order to facilitate voltage contrast analysis there must be a charging difference between the defective structure and its surroundings.
Typically the sub-surface structure is electrically connected to the substrate of the wafer or is otherwise connected to an external voltage source or to the ground. Thus, the charging of the structure surrounding can be relatively easily controlled.
In order to perform an efficient voltage contrast measurement it is necessary to develop a substantial difference between the potential of the defective structure and the non-detective structures. In particular, to enable signal separation via energy filtering it is necessary to develop a difference in potential that is greater than the nominal energy width of the secondary electrons of the materials comprising the defective and non-defective structures.
Various techniques are known for performing voltage contrast measurements. A first technique includes using a flooding gun. The sample can be moved towards the flooding gun that directs a relatively large amount of electrons towards a predefined area, thus charging the area. The area is then scanned, by a scanning beam, in order to provide an image of that area. Another technique involves providing a bias voltage to the sample. Combinations of these techniques are also known. An example of a combination of both techniques is described in U.S. Pat. No. 6,828,571 to McCord et al., entitled “Apparatus and methods of controlling surface charge and focus”. McCord also describes an auto-focus method that involves illuminating the sample with a tilted monitor beam.
Some inspection devices and methods scan the sample by an electron beam that is focused to a small spot-size such as is necessary to obtain the desired resolution. The diameter of the spot typically needs to be less than twice the desired resolution. For state-of-the-art semiconductor wafers this typically imposes the requirement that the spot size be less than 500 nm.
Typically, the electrical beam is scanned along a scan axis while the sample is mechanically shifted along a transverse axis. Thus, repetitive exposures of the same area are relatively avoided.
Many types of scanning electron microscopes are known, typically including an electron gun directing an electron beam at a target via a vacuum column comprising a condensing lens, scan coils, and an objective lens; a detector and associated amplifier receiving electrons emitted from the target, and a monitor.
An “octupole”, according to Wikipedia, typically refers to “two electric or magnetic quadrupoles having charge distributions of opposite signs and separated from each other by a small distance” and/or “any device for controlling beams of electrons or other charged particles, consisting of eight electrodes or magnetic poles arranged in a circular pattern, with alternating polarities; commonly used to correct aberrations of quadrupole systems.”
The SemVision/G2 is a commercially available system which employs, among other methods, a ‘delta-charging’ technique.
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